<button (click)="showJavaArticles()">Java Articles</button>
<button (click)="showAngularArticles()">Angular Articles</button>
<button (click)="showFavoriteArticles()">Favorite Articles</button>
<ul>
   <li *ngFor="let article of articles | async">
        {{article.id}} - {{article.title}} - {{article.category}}   
   </li>
</ul>